1. Field of the Invention
The present invention relates to a control device for maintaining a constant speed of an automobile having an internal combustion engine.
2. Description of the Related Art
The speed of an automobile is generally controlled by operating an accelerator pedal mechanically connected to a throttle valve arranged in the internal combustion engine of the automobile. Therefore, it is obviously possible to control the speed of the automobile by actuating the throttle valve by a means other than the accelerator pedal. Recently, an automobile autocruising device has been developed which allows a driver to automatically drive the automobile, while releasing the foot from the accelerator pedal, by the provision of an actuator for the throttle valve, and which fundamentally includes a control device for maintaining a constant speed of the automobile.
The speed of the automobile has a linear relationship to a degree of opening of the throttle valve, as shown, for example, in FIG. 7 of the attached drawings, while the automobile is running on an even road. Therefore, it is possible to decide a degree of opening of the throttle valve ST.sub.O according to a desired set speed SPM, if given, and thus it is possible to cause the automobile to cruise constantly at the desired set speed SPM by constantly maintaining that degree of opening of the throttle valve ST.sub.O.
However, when the automobile is running on a slope or travelling uphill, as shown, for example, in FIG. 8 of the attached drawings, the speed of the automobile may be lowered if the degree of opening of the throttle valve is maintained at a constant value. Therefore, it is necessary to increase the degree of opening of the throttle valve to a value ST.sub.1, as shown in FIG. 7, in order to maintain a constant speed of the automobile. However, it is impossible to memorize all roads and other running conditions of the automobile, and thus it becomes necessary to regulate the degree of opening of the throttle valve while the automobile is running.
Accordingly, it becomes necessary to correct the degree of opening of the throttle valve in accordance with actual running conditions, to enable the automobile to cruise at a constant speed. For this purpose, an integral correction calculation is conventionally used, which comprises a calculation of an integral correction value to correct the degree of opening of the throttle valve when there is a difference between the actual speed and the desired set speed, by cyclically repeating an integral calculation in a predetermined pattern. For example, the following calculation is repeatedly carried out. EQU TON=TON+CG(SPM-SPD)
where,
TON=integral correction value of a degree of opening of the throttle valve PA1 SPM=set speed PA1 SPD=actual speed PA1 CG=correction gain
This calculation is cyclically repeated according to a computer operating cycle (for example, every 50 ms) to integrate or accumulate a value CG(SPM - SPD) which corresponds to the deviation in the speed of the automobile.
It will be appreciated that, when the automobile is running on a long upward slope, as shown in (A) of FIG. 8, the actual speed SPD becomes lower than the set speed SPM, as shown in (C) of FIG. 8, so that a value corresponding to the speed deviation CG((SPM -SPD) is integrated many times in sequence while the actual speed SPD is lower than the set speed SPM, and the integral correction value TON continues to increase, as shown in FIG. (B) of FIG. 8.
When the automobile reaches the top of the slope after the long uphill run, the road then may be flat or run downhill. In this situation, the integral correction value TON, and accordingly, the degree of the opening of the throttle valve, will become a greater value and thus the speed of the automobile will be allowed to increase. Therefore, it is necessary to change the integral correction value TON and the degree of the opening of the throttle valve. Also, in this case, since the above formula TON=TON+CG(SPM-SPD) has been used, and the deviation CG((SPM-SPD) has become a negative value, the integral correction value TON is reduced to a smaller value.
In this integral correction calculation, the correction gain CG cannot be made a relatively large value, to avoid an excess control or undesirable variations on a flat road, and in practice a very small value is used. Accordingly, the rate of change of the integral correction value TON is very small, so that an undershoot in the speed of the automobile at the initial stage of entering the uphill road and an overshoot on the change from the uphill run to the downhill run will occur, as shown in (C) of FIG. 8. This problem can be solved to a great extent by shortening the computer operating cycle to shorten the control response time in accordance with the running conditions, even if the correction gain CG is very small. However, a serious problem still remains when the automobile has run on a very long upward slope and the integral correction value TON has become a very large value because of repeated increases in the calculation over a long period, even if the correction gain CG is very small. In many cases, the integral correction value TON must be made zero or even a minus value immediately after the automobile reaches the top of the hill or passes across the top of the hill to the downhill run, but a considerable time is needed to return the highly integrated correction value TON to a lower value, as above stated, caused by the long uphill ascending time, since the calculation is carried out in the same manner throughout the running of the automobile.
Japanese Patent Application Nos. 60-298132 and 60-294227, filed jointly by the assignee for the present case and a Japanese corporation, disclose a control device for maintaining a constant speed of an automobile by using a vacuum operated actuator which is operated under a duty control. This control device also uses an integral correction calculation, similar to the above described device, but more particularly, comprises a first integral correction element providing a quick response to a change in running conditions and a second integral correction element provided a relatively lower response to such a change. These elements are appropriately combined or selectively used to increase the response at the start of an upward slope and downward slope, or on an undulating road, and to decrease the variation in the speed on a flat road.